The use of hydrofoils on relatively large craft has been known for some years. The purpose of such foils is to allow a vessel in which they are attached to ride with its hull above the water surface, thus reducing drag and therefore power requirements.
There have been recent attempts to apply this technology to small craft such as pleasure boats. Fixed hull-mounted foils are unsuitable for most small craft because they require a considerably increased draft for docking and low speed operation. This has led to the use of rather complex and expensive arrangements for retracting and extending such foils.
There have also been attempts to employ hydrofoils giving limited lift to improve the planing characteristics of small boats. Reference may be made, for example, to U.S. Pat. No. 3,433,195 which discloses a fixed foil attachment mounted on the propulsion unit of an outboard motor. The difficulty with an arrangement of this sort is that, while it may operate well during normal forward motion of the boat, it can have detrimental effects on the behaviour of the craft at other times. Thus, if the boat is driven in reverse by reversing the drive of the motor, the angle of attack of the foils will be negative, and this will tend to draw the stern of the boat down into the water. During forward motion, if the propulsion unit impacts on an underwater obstacle and pivots upwardly to the rear, the foils again adopt a negative angle of attack, drawing the stern of the boat downwardly and generating a strong decelerating force which chould be sufficiently strong as to have a destructive effect on the transom of the boat or the propulsion unit itself.